Define nxt_cpu_pause for ARM64.

The isb instruction fits for spin loops where it allows to save cpu
power.

Reviewed-by: Andrew Clayton <a.clayton@nginx.com>
Signed-off-by: Andrew Clayton <a.clayton@nginx.com>

src/nxt_atomic.h

@@ -58,6 +58,10 @@ typedef volatile nxt_atomic_uint_t  nxt_atomic_t;
 #define nxt_cpu_pause()                                                       \
     __asm__ ("pause")
 
+#elif (__aarch64__ || __arm64__)
+#define nxt_cpu_pause()                                                       \
+    __asm__ ("isb")
+
 #else
 #define nxt_cpu_pause()
 #endif